This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the presently described embodiments. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present embodiments. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Drilling and production operations for the recovery of offshore deposits of crude oil and natural gas are taking place in deeper and deeper waters. Drilling and production operations in deeper waters are typically carried out from floating vessels rather than from stationary platforms resting on the ocean floor and commonly used in shallow water. According to conventional procedures, a vessel is dynamically stationed, or moored, above a well site on the ocean floor. After a wellhead has been established, a blowout preventer (“BOP”) stack including one or more BOPs is mounted on the wellhead to control the pressure in the wellhead.
Typical BOPs are used as a large specialized valve or similar mechanical device that seal, control, and monitor oil and gas wells. The two most common categories of BOPs are ram BOPs and annular BOPs. BOP stacks frequently utilize both types of BOPs, typically with at least one annular BOP stacked above several ram BOPs. The ram units in ram BOPs allow for shearing drill pipe in the case of shear rams, sealing off around drill pipe in the case of pipe rams, and sealing the BOP bore in the case of blind rams. Typically, a BOP stack may be secured to a wellhead and may provide a safe means for sealing the well in the event of a system failure.
An example BOP includes a main body or housing with a vertical bore. Rain bonnet assemblies may be bolted to opposing sides of the main body using a number of high tensile fasteners, such as bolts or studs. These fasteners are required to hold the bonnet in position to enable the sealing arrangements to work effectively. An elastomeric sealing element may be used between the ram bonnet and the main body. There are several configurations, but essentially they are all directed to preventing a leakage bypass between the mating faces of the ram bonnet and the main body.
Each bonnet assembly includes a piston which is laterally movable within a ram cavity of the bonnet assembly by pressurized hydraulic fluid acting on the piston. The opposite side of each piston has a connecting rod attached thereto which in turn has a ram mounted thereon for extension into the vertical bore. The rams can be shear rams for shearing an object within the bore of a BOP. Alternatively, the rams can be pipe rams for sealing off around an object within the bore of a BOP, such as a pipe, thereby sealing the annular space between the object and the BOP bore. In addition, the rams can be blind rams for sealing the BOP bore.
The rams are designed to move laterally toward the vertical bore of the BOP to shear or seal off on any object located therein. For instance, opposing shear rams utilize cutting surfaces configured to close in on and shear an object located in the BOP bore, such as a section of drill pipe used during drilling operations. The opposing shear rams can include seals disposed adjacent the cutting faces of the shear rams and configured to come together to seal off the BOP bore.
Pipe rams utilize seals that close in on and seal off on a tubular within the vertical bore of the BOP. Each pipe ram typically has a semicircular opening in its front face to form a seal about half of the outer periphery of the object within the BOP vertical bore. When the opposing pipe rams are closed, the opposing pipe rams engage each other and seal the entire periphery of the object, thereby closing off the annulus between the object and the BOP bore. Typical pipe ram assemblies can include a ram packer which is composed of an elastomeric or rubber material configured to seal off against the tubular within the vertical bore of the BOP when the opposing rams are run into the closed position.
Still further, blind rams utilize seals that close in and seal on opposing blind ram seals to seal off a BOP bore when no object is present in the bore.
Each ram-type BOP within a BOP stack further includes one or more access ports (e.g., choke or kill ports) located below the associated ram cavities. The access ports are located on either side of the BOP body and are configured to provide a conduit for pumping fluid into or out of the wellbore while the associated rams are closed and in the event of a well kick. The fluid flow through the access port is handled by choke or kill lines, which can extend from the surface to the subsea BOP housing.
The access ports extend laterally through the BOP housing and are in fluid communication with the bore of the BOP. Typically, there are two access ports located within the BOP body and below the ram cavity of each BOP. In practice, only one access port located below a ram cavity is actively associated with a choke or kill line. The other access port under the particular ram cavity is generally unused during well operations.
Monitoring the opening and closing performance of rams is critical in understanding the ability of a BOP to close in on and seal off a BOP wellbore. Current methods for monitoring the position of the rams, for instance, during closing operations, includes monitoring the position of the piston driving the rams. By monitoring the position of the piston driving the ram, the general position of the ram can be estimated. Other methods include monitoring the amount of hydraulic fluid used to close a piston. Based on the volume of hydraulic fluid used, the displacement of the piston and ram can be estimated. However, other methods of monitoring the position of rams in a BOP are desirable. In particular, utilizing an existing but unused access port already located on a BOP housing (e.g., choke or kill port) to monitor the position of rams within a BOP is particularly desirable.